In Python, len
is a function to get the length of a collection by calling an object’s __len__
method:
def len(x): return x.__len__()
So I would expect direct call of __len__()
to be at least as fast as len()
.
import timeit setup = ''' ''' print (timeit.Timer('a="12345"; x=a.__len__()', setup=setup).repeat(10)) print (timeit.Timer('a="12345"; x=len(a)', setup=setup).repeat(10))
But results of testing with the above code shows len()
to be faster. Why?
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Answer
The builtin len()
function does not look up the .__len__
attribute. It looks up the tp_as_sequence
pointer, which in turn has a sq_length
attribute.
The .__len__
attribute on built-in objects is indirectly mapped to the same slot, and it is that indirection (plus the attribute lookup) that takes more time.
For Python-defined classes, the type
object looks up the .__len__
method when the sq_length
is requested.